The German will continue on Thursday alongside Brazilian Antonio Pizzonia, the Williams test driver who is the favourite to step up to the race seat.

Heidfeld had been due to test next week but was called up early to take the place of Briton Anthony Davidson, whose appearance was blocked by rivals BAR.

Williams will have a new line-up next year after Germany's Ralf Schumacher and Colombia's Juan Pablo Montoya departed to Toyota and McLaren respectively.

Brazilian Felipe Massa, in a Sauber, was fastest in the Jerez tests due to be attended by all of the 10 teams except Minardi this week.

Jordan, testing for the first time since the end of the season in October, went double Dutch with Christijan Albers and Robert Doornbos.

BAR tried out South Africa's Alan van der Merwe, last year's winner of the junior British Formula Three championship, with Briton Adam Carroll.

Red Bull Racing, who have replaced Jaguar and have yet to confirm their 2005 line-up, continued assessing Italian Vitantonio Liuzzi with Austrian Christian Klien.

The Austrian energy drink-owned team said they would also try out 21-year-old Swiss Neel Jani, another Red Bull sponsored driver who has done some testing for Sauber over the last two years, on Friday.